Air kerma conventional true value measuring method

ABSTRACT

A measurement method for an air kerma conventional true value comprises: building a small-scale reference radiation field, then selecting a proper radiation source ( 4 ) and a source intensity for providing incident rays for a shielding box ( 1 ), subsequently selecting a plurality of gamma ray dose measurement instruments as experiment samples for building a prediction model to obtain a prediction model of the air kerma conventional true value of a check point, fmally placing a probe of an instrument to be detected on the check point ( 6 ), recording a scattering gamma spectrum detected by a gamma-ray spectrometer ( 9 ), and importing the prediction model to obtain the air kerma conventional true value. The method relates to the field of radiation protection detection or calibration, and has the beneficial effects that the result is accurate, the reference radiation field used is small in size, and the method is applied to measurement of the air kerma conventional true value. The method solves the problem that site and in-situ detection or calibration is unlikely to be implemented as the existing standard reference radiation field is too large in space and volume to move or is difficult to move.

TECHNICAL FIELD

The present invention relates to the field of verification orcalibration of radiation protection, and particularly relates to an airkerma conventional true value measuring method.

BACKGROUND OF THE INVENTION

Instrumentation for measuring gamma ray dose (rate) and dose equivalent(rate) is widely applied in military, national defense and civil fields,is extremely important toolbox means for guaranteeing the safety ofnuclear facilities, gamma ray devices, working personnel and the public,and plays an indispensable role. To ensure the accuracy and reliabilityof performance and measured values thereof, Metrology Law andcorrelative regulations in our country prescribe that they should beverified or calibrated periodically and are measuring instruments to beverified compulsorily.

Gamma dose (rate) instrumentation should be verified and calibrated ongamma air kerma secondary standard devices containing secondary standardreference radiation fields constituted by isotope radiation sourcesaccording to the requirements of the national standard GB/T 12162.1-2000“X and gamma reference radiation for calibrating dose meters and doserate meters and for determining their response—Radiation characteristicsand production methods”, GB/T12162.2-2004 “X and gamma referenceradiation for calibrating dose meters and dose rate meters and fordetermining their response, Part 2: Dosimetry for radiation protectionover the energy ranges 8 keV to 1.3 MeV and 4 MeV to 9 MeV”, andJJG393-2003 “Verification Regulation of X and Gamma Radiation DoseEquivalent (Rate) Meters and Monitors Used in Radiation Protection”. Inthe verification and calibration work, a secondary standard referenceradiation field is required to be verified first by using an air kermameasurement standard instrument to obtain an air kerma conventional truevalue at a check point of the secondary standard reference radiationfield; then a reference point on a probe of the instrument beingdetected is accurately positioned in the secondary standard referenceradiation field as required, and measurement is performed to obtain acalibration factor K:

${K = \frac{{\overset{.}{K}}_{{air},c}}{{\overset{.}{M_{c}}}^{\prime}}},$

wherein, I_(—air,c) {dot over ( )} is the gamma air kerma (rate)measured or calculated by the standard instrument at an experiment pointof the secondary standard reference radiation field, i.e., aconventional true value of gamma air kerma (rate) at the experimentpoint, and

${\overset{.}{M}}_{c}$

is an indicating value of an instrumentation being detected.

When a gamma air kerma secondary standard device is constructed, whetherthe standard reference radiation constructed meets the requirement isverified by scientific design, detailed experimental test and correctionon the size of a reference radiation field influencing the dose value,scattering of a shielding wall and ground and the non-uniformity of anirradiation area of ray beams and the radiation field and the like. Inaccordance with relevant standards, the spatial volume of the standardreference radiation field meeting the above requirement shall not besmaller than 4 m×4 m×3 m, and the dose rate of gamma rays of an isotoperadiation source shall cover the range from μGy/h to mGy/h. Suchstandard reference radiation cannot be moved no matter in volume or inweight including a shielding building or the like, which leads that allgamma ray radiation protection instrumentation must be delivered tometrology technology institutions possessing standard referenceradiation fields at fixed sites for verification or calibration.Instrumentation for the purposes of radiation safety surveillance onnuclear power plant reactors and relevant nuclear facilities, becausethey are impossible or difficult to be detached, have not beenperiodically verified or calibrated by scientific method andtechnologies and proper device means, thus burying hidden danger forradiation safety.

One way to realize on-site and in-situ verification or calibration on agamma ray dose measuring instrument is to reduce the spatial volume andthe weight of the at least 4 m×4 m×3 m standard reference radiationfield prescribed by the standards till movement is facilitated. However,the reduction of the spatial volume of the reference radiation fieldinevitably leads to increase of scattering components in the radiationfield, so that the dose contribution rate of scattered rays in the smallreference radiation field is more than 5%, which does not comply withthe requirements of existing standards, influences the response of theinstrument and results in deviation of the calibration coefficient.

SUMMARY OF THE INVENTION

The present invention is aimed at providing an air kerma conventionaltrue value measuring method for overcoming the defect that the on-siteand in-situ verification or calibration is relatively difficult as theexisting standard reference radiation field is large in spatial volumeand is impossible or difficult to be moved.

The technical solution adopted by the invention to solve the technicalproblems thereof is an air kerma conventional true value measuringmethod, which is characterized by including the following steps:

step 1, establishing a small-scale reference radiation field (MRR), thesmall-scale reference radiation field comprising a shielding box havinga side length not more than 1.5 meters, the shielding box beingpositioned horizontally and an incident hole being provided on the sidethereof for incidence of incident rays, a check point being arranged inthe direction of the incident rays in the shielding box, the shieldingbox being further provided with a test hole on the upper surface throughwhich a probe of an instrument to be detected can be put into theshielding box, a reference point on the probe being superposed with thecheck point, a dose feature point being also arranged in the shieldingbox, the shielding box being segmented into two parts by one planeperpendicular to the connecting line of the incident hole and the checkpoint and containing the check point, the dose feature point beinglocated at the part close to the incident hole in the shielding box andat the position not directly irradiated by the incident rays, a gammaspectrometer being arranged in the shielding box, a reference point on aprobe thereof being superposed with the dose feature point and the probebeing fixed in the shielding box;

step 2, selecting a proper radiation source and source strength toprovide incident rays for the shielding box;

step 3, selecting a plurality of gamma ray dose measurement instrumentsas experimental samples for establishing a prediction model to obtainthe prediction model of the air kerma conventional true value at thecheck point; and

step 4, putting the probe of the instrument to be detected at the checkpoint, recording scattering gamma spectra measured by the gammaspectrometer, and introducing the scattering gamma spectra to theprediction model to obtain an air kerma conventional true value.

Specifically, step 3 includes the following specific steps:

step 31, selecting a plurality of gamma ray dose measurement instrumentsas experimental samples for establishing a prediction model;

step 32, measuring the air kerma conventional true value at the checkpoint when no experimental sample is put, then putting a reference pointof a probe of an experimental sample on the check point, measuring theair kerma conventional true value at the check point by adopting aninstrument transfer method, and acquiring gamma energy spectra of thedose feature point via the gamma spectrometer;

step 33, acquiring a dose feature value by adopting a principalcomponent analysis method according to the gamma energy spectra; and

step 34, obtaining a prediction model of the air kerma conventional truevalue at the check point by adopting a support vector machine regressionmethod.

Further, step 32 includes the following specific steps:

step 32A, putting a standard graphite cavity ionization chamber at thecheck point, and measuring the air kerma conventional true value K_(j)′at the check point when the strength of an incident ray beam is V_(j);

step 32B, putting the reference point of the probe of the i^(th)experimental sample at the check point, setting the strength of theincident ray beam as V_(j), recording the reading of the experimentalsample R_(ij) and obtaining the gamma energy spectrum S_(ij) of the dosefeature point at the moment via the gamma spectrometer;

step 32C, putting the experimental sample in the standard referenceradiation field to look for a point having the reading equal to R_(ij),the corresponding air kerma conventional true value of the point beingthe air kerma conventional true value K_(ij) at the check point;

step 32D, sequentially putting the reference points of the probes of thex experimental samples in the check point, and repeating steps 32A to32C under y source strength conditions to obtain x×y groups of K_(ij),S_(ij) and K_(j)′ data for establishing a model of a functionrelationship K_(ij)=f₁(S_(ij),K_(j)′).

Specifically, step 33 includes the following steps:

step 33A, scattering each acquired S_(ij) according to a certain energyinterval AE to obtain a counting rate η_(ijn) array corresponding to theenergy of the scattering gamma ray, and constructing n-dimensionalvectors a_(ij) of the counting rates using the energy of the scatteringgamma ray as a research object;

step 33B, constructing a scattering gamma energy spectrum counting ratedata matrix sample Φ_((x×y)×n) via the experiments of the probes of thex experimental samples under the y source strength conditions in step32D;

step 33C, solving principal components of the n-dimensional vectorsa_(ij) by adopting a principal component analysis method to obtainprincipal component vectors ψ_(ij)=T_(n×m) ^(T)·a_(ij) of then-dimensional vectors a_(ij), m≦n, T_(n×m) ^(T) being a transposition ofT_(n×m), and T_(n×m) referring to obtaining a covariance matrix ξ_(n×n)from Φ_(X×y)×n); and solving a score matrix composed of m first featurevectors of the covariance matrix ξ_(n×n); and

step 33D, obtaining a function relationship ψ_(ij)=f₂(S_(ij)) betweenψ_(ij) and S_(ij), thus obtainingK_(ij)=f₃(ψ_(ij),K_(j)′)=f₃[f₂(S_(ij)),K_(j)′].

Further, in step 33A, the certain energy interval ΔE refers to:

ΔE=1500/(128×2^(z))keV, 0≦z≦4,

z being an integer.

Specifically, step 33C includes the following specific steps:

step 33C1, obtaining a covariance matrix ξ_(n×n) from Φ(_(x×y))×_(n),and solving n feature values λ₁≧λ₂≧ . . . ≧λ_(n)≧0 of the covariancematrix ξ_(n×n) and corresponding feature vectors t₁, . . . t_(m), . . .t_(n);

step 33C2, obtaining a score matrix T_(n×m)=(t₁, . . . , t_(m)) of theprincipal components, wherein m is determined by formula Σ_(k=1)^(m)λ_(k)/Σ_(k=1) ^(n)λ_(k)≧δ_(m)≧85%; and

step 33C3, obtaining the principal component vectors ψ_(ij)=T_(n×m)^(T)·a_(ij) of the n-dimensional vectors a_(ij), wherein m≦n, andT_(n×m) ^(T) is a transposition of T_(n×m).

Still further, step 34 includes the following steps:

step 34A, obtaining a data matrix sample(K_(ij),ψ_(ij),K_(j)′)_((x×y)×(m+2)) via the experiments of the probesof the x experimental samples under the y kinds of V_(j) conditions instep 32D, and obtaining a prediction model K_(ij)=f₃(ψ_(ij),K_(j)′) ofK_(ij) by adopting a support vector machine regression method.

Specifically, in step 34A, the specific method of obtaining a predictionmodel K_(ij)=f₃(ψ_(ij), K_(j)′) of K_(ij) by adopting a support vectormachine regression method includes the steps of: training a kernelfunction selected by a regression prediction model as a radial basiskernel, the parameter of the kernel function being determined by a crossvalidation method; when the model is established, allocating the datasamples (K_(ij),ψ_(ij),K_(j)′)_((x×y)×(m+2)) to a training set and atest set according to a certain proportion; and when the test error isnot more than 5%, ending the training, and determining the predictionmodel K_(ij)=f₃(ψ_(ij),K_(j)′).

Still further, the certain proportion refers to that the proportion ofthe training set to the test set is more than or equal to 1:1.

Specifically, step 4 includes the following steps:

step 41, putting the reference point of the probe of the instrument tobe detected at the check point;

step 42, selecting a proper radiation source and putting it into anisotope radiation source accommodating device, and adjusting anattenuator to obtain the measured strength V_(j) of the incident raybeam; and

step 43, measuring the scattering gamma spectrum at the moment by usingthe gamma spectrometer, and introducing the scattering gamma spectrumdata into the prediction model establishedK_(ij)=f₃(ψ_(ij),K_(j)′)=f₃[f₂(S_(ij)),K_(j)′]=f₁(S_(ij),K_(j)′) toobtain an air kerma conventional true value at the check point.

The beneficial effects of the present invention are that a small-scalereference radiation field is adopted by the aforesaid air kermaconventional true value measuring method; because when the predictionmodel K_(ij)=f₁(S_(ij),K_(j)′) is established, the dose featureinformation represented by the probes of the gamma dose measurementinstruments (experimental samples) in the MRR and the scattering gammaspectra are extracted reasonably by the method of the present inventionthrough using the PCA method, meanwhile the data volume of the gammaspectrum for modeling is greatly reduced, and the calculation speed ofmodeling and prediction is improved; by selecting the SVM suitable forthe multivariate linear regression under the small sample condition, theestablished prediction model has good compatibility and good promotionprediction capability for the types of radiation sources and detectors,the compatibility of the model is good, the prediction result isaccurate, and the interference of complex scattering gamma rays causedby disturbance of small-scale reference radiation and detectors on theradiation field to measurement of the air kerma conventional true valueat the check point in the MRR is successfully solved. The measured airkerma conventional true value is equivalent to national standards GB/T12162.1-2000, GB/T 12162.2-2004 and JJG393-2003, and the method is thussuitable for verification or calibration of the aforesaid radiationprotection gamma ray dose (rate) and dose equivalent (rate) instrumentsrequired by the standards. By using the method of the present invention,devices and equipment having the weight and volume suitable forverification or calibration of mobile skid-mounted, vehicle-mounted,hand-propelled or other mobile gamma ray radiation protectioninstruments can be designed and manufactured for verification andcalibration of using sites of various gamma ray dose measurementinstruments and safety surveillance instruments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structural schematic diagram of a small-scale referenceradiation field in an embodiment of the present invention;

FIG. 2 is a scattering gamma energy spectrum diagram of a gamma doseinstrument BH3103A in MMR at a feature dose point in the embodiment ofthe present invention;

FIG. 3 is a schematic diagram of a linear combination coefficient ofprinciple components of scattering gamma energy spectra at the featuredose point in the embodiment of the present invention.

Among them, 1 is a shielding box, 2 is an instrument to be detected, 3is incident ray, 4 is a radiation source, 5 is a test hole, 6 is a checkpoint, 7 is a dose feature point, 8 is an incident hole, and 9 is agamma spectrometer.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The technical solution of the present invention will be described indetail below in combination with an embodiment and the accompanyingdrawings.

An air kerma conventional true value measuring method of the presentinvention is that: establishing a small-scale reference radiation field(MRR) first, the small-scale reference radiation field comprising ashielding box and a gamma spectrometer, the shielding box beingpositioned horizontally and provided with an incident hole on the sidethe shielding box for the incidence of incident rays, a check pointbeing arranged in the direction of the incident rays in the shieldingbox, the shielding box being further provided with a test hole on theupper surface through which a reference point on a probe of aninstrumentation to be detected can be superposed with the check point, adose feature point being further arranged in the shielding box, theshielding box being segmented into two parts by one plane that isperpendicular to the connecting line of the incident hole and the checkpoint and that contains the check point, the dose feature point beinglocated at the part close to the incident hole in the shielding box andat the position not directly irradiated by the incident rays, a gammaspectrometer being arranged in the shielding box, the reference point ona probe of the gamma spectrometer being superposed with the dose featurepoint and the probe being fixed in the shielding box; then selecting aproper radiation source for providing incident rays for the shieldingbox, selecting a plurality of gamma ray dose measurement instruments asexperimental samples for establishing a prediction model to obtain aprediction model of the air kerma conventional true value at the checkpoint, and finally during a test: putting the probe of the instrument tobe detected at the check point, recording the scattering gamma spectrameasured by the gamma spectrometer, and introducing the prediction modelto obtain an air kerma conventional true value.

The Embodiment

In this embodiment, the structural schematic diagram of the small-scalereference radiation field (MRR) is shown as FIG. 1, and the small-scalereference radiation field comprises a shielding box 1 having a sidelength not more than 1.5 meters and a gamma spectrometer 9. Theshielding box 1 is positioned horizontally and provided with an incidenthole 8 on the side thereof for the incidence of incident rays 3. A checkpoint 6 is arranged in the direction of the incident rays 3 in theshielding box 1, a test hole 5 is further provided on the upper surfaceof the shielding box 1, a probe of an instrumentation 2 to be detectedcan be put into the shielding box 1 via the test hole 5 to make areference point on the probe being superposed with the check point 6. Adose feature point 7 is further arranged in the shielding box 1, theshielding box 1 is segmented into two parts by one plane that isperpendicular to the connecting line of the incident hole 8 and thecheck point 6 and that contains the check point 6, and the dose featurepoint 7 is located at the part close to the incident hole 8 in theshielding box 1 and at the position not directly irradiated by theincident rays 3. The gamma spectrometer 9 is arranged in the shieldingbox 1, a reference point on a probe thereof is superposed with the dosefeature point 7, and the gamma spectrometer 9 is fixed in the shieldingbox 1.

In this embodiment, the shielding box 1 may be a cube having a sectionalsize of 1 meter, e.g., a 1 m×1 m×1 m sized cube, and may also be acuboid or in other shape, the specific size being determined by thetotal weight of the MRR allowed by the intended use. The incident hole 8may be located in the center position of the side of the shielding box,the check point 6 may also be located in the geometrical center of theshielding box, and the dose feature point 7 is generally located on theinner bottom of the shielding box 1.

In use, the specific method includes the following steps:

Step 1, constructing the aforesaid small-scale reference radiation fielddevice.

Step 2, selecting a proper radiation source and source strength forproviding incident rays for the shielding box.

Step 3, selecting a plurality of gamma ray dose measurement instrumentsas experimental samples for establishing a prediction model to obtainthe prediction model of the air kerma conventional true value at thecheck point.

This step includes the following specific steps:

step 31, selecting a plurality of gamma ray dose measurement instrumentsas experimental samples for establishing a prediction model, and theplurality of selected gamma ray dose measurement instruments may be thegamma ray dose measurement instruments of types BH3103A, FJ317E, SSM-1,FD-3013B, CIT-2000FX.γ, Inspector1000 and Canberra Radiagem2000;

step 32, measuring the air kerma conventional true value at the checkpoint when no experimental sample is arranged, putting a reference pointof a probe of an experimental sample on the check point, measuring theair kerma conventional true value at the check point by adopting aninstrument transfer method, and acquiring gamma energy spectra of thedose feature point via the gamma spectrometer. The specific method is asfollows:

step 32A, putting a standard graphite cavity ionization chamber at thecheck point, and measuring the air kerma conventional true value at thecheck point, denoted as K_(j)′, when the strength of an incident raybeam is V_(j);

step 32B, putting the reference point of the probe of the i^(th)experimental sample at the check point, setting the strength of theincident ray beam as V_(j), recording the reading of the experimentalsample R_(ij) and obtaining the gamma energy spectrum of the dosefeature point at the moment via the gamma spectrometer S_(ij);

step 32C, putting the experimental sample in the standard referenceradiation field to look for a point having the reading equal to R_(ij),the air kerma conventional true value corresponding to the point beingthe air kerma conventional true value at the check point K_(ij);

step 32D, sequentially putting the reference points of the probes of thex experimental samples in the check point, repeating steps 32A to 32Cunder y kinds of Vj conditions to obtain x×y groups of K_(ij), S_(ij)and K_(j)′ data, and obtaining a function relationship K_(ij)=f₁(S_(ij),K_(j)′) thereof;

step 33, acquiring a dose feature value by adopting a principalcomponent analysis method according to the gamma energy spectrum. Thespecific method is as follows:

step 33A, scattering each acquired S_(ij) according to a certain energyinterval ΔE to obtain a counting rate η_(ijn) array corresponding to theenergy of the scattering gamma ray, and constructing n-dimensionalvectors a_(ij) of the counting rates using the energy of the scatteringgamma ray as a research object; here, the certain energy interval ΔErefers to ΔE=1500/(128×2^(z))keV, and z is an integer more than or equalto 0 and less than or equal to 4;

step 33B, constructing a scattering gamma energy spectrum counting ratedata matrix sample Φ_((x×y)×n) via the experiments of the probes of thex experimental samples under the y kinds of V_(j) conditions in step32D;

step 33C, solving the principal components of the n-dimensional vectorsa_(ij) by adopting a principal component analysis method to obtainprincipal component vectors ψ_(ij)=T_(n×m) ^(T)·a_(ij) of then-dimensional vectors a_(ij), wherein m≦n, T_(n×m) ^(T) is atransposition of T_(n×m), and T_(n×m) refers to obtaining a covariancematrix ξ_(n×n) from Φ_((x×y)×n); and solving a score matrix composed ofm first feature vectors of the covariance matrix ξ_(n×n). The specificmethod is as follows:

step 33C1, obtaining a covariance matrix ξ_(n×n) from Φ_((x×y)×n), andsolving n feature values λ₁≧λ₂≧ . . . ≧λ_(n)≧0 of the covariance matrixξ_(n×n) and corresponding feature vectors t₁, . . . , t_(m), . . .t_(n);

step 33C2, a score matrix of the principal components is T_(n×m)=(t₁, .. . , t_(m)), wherein m is determined by the formula Σ_(k=1)^(m)λ_(k)/Σ_(k=1) ^(n)λ_(k)≧δ_(m), and δ_(m)≧85%;

step 33C3, the principal component vectors of the n-dimensional vectorsa_(ij) is ψ_(ij)=T_(n×m) ^(T)·a_(ij), wherein m≦n, and T_(n×m) ^(T) is atransposition of T_(n×m);

step 33D, obtaining a function relationship ψ_(ij)=f₂(S_(ij)) betweenψ_(ij) and S_(ij), thus obtainingK_(ij)=f₃(ψ_(ij),K_(j)′)=f₃[f₂(S_(ij)),K_(j)′];

step 34, obtaining regressionally a prediction model of the air kermaconventional true value at the check point by adopting a support vectormachine method. The specific method is as follows:

step 34A, obtaining a data matrix sample(K_(ij),ψ_(ij),K_(j)′)_((x×y)×(m+2)) via the experiments of the probesof the x experimental samples under the y kinds of V_(j) conditions instep 32D, and obtaining a prediction model K_(ij)=f₃(ψ_(ij),K_(j)′) ofK_(ij) by adopting a support vector machine regression method. Thespecific method is as follows: training a kernel function selected by aregression prediction model as a radial basis kernel, the parameter ofthe kernel function is determined by a cross validation method; when themodel is established, allocating the data samples(K_(ij),ψ_(ij),K_(j)′)_((x×y)×(m+2)) to a training set and a test setaccording to a certain proportion; and when the test error is not morethan 5%, ending the training, and determining the prediction model asK_(ij)=f₃(ψ_(ij),K_(j)′). Herein, the certain proportion refers to thatthe proportion of the training set to the test set is more than or equalto 1:1.

Step 4, putting the probe of the instrument to be detected at the checkpoint, recording scattering gamma spectra measured by the gammaspectrometer, and introducing the scattering gamma spectra to theprediction model to obtain an air kerma conventional true value.

Step 4 includes the following specific steps:

step 41, putting the reference point of the probe of the instrument tobe detected at the check point;

step 42, selecting a proper radiation source and putting it into anisotope radiation source accommodating device, and adjusting anattenuator to obtain the strength V_(j) of the measured incident raybeam; and

step 43, measuring the scattering gamma spectrum at the moment by usingthe gamma spectrometer, and introducing the scattering gamma spectrumdata into the established prediction modelK_(ij)=f₃(ψ_(ij),K_(j)′)=f₁(S_(ij),K_(j)′) to obtain an air kermaconventional true value at the check point.

The methohd may further include the following step:

obtaining a correction factor ω=K_(ij)/R_(ij) in combination with thereading of the instrument to be detected R_(ij).

The energy response characteristic of the instrument to be detected canalso be obtained via the above method by switching the radiation sourceswith different energy; or the angle response data of the instrument tobe detected can be obtained by rotating the probe of the instrument tobe detected, and other verification items stipulated in JJG393-2003 canalso be realized.

A specific example is as follows:

A radioisotope source ¹³⁷Cs is selected as the radiation source 4 inthis embodiment to provide a radiation ray source for the small-scalereference radiation (MRR), and a calibration device for calibration of agamma ray radiation protection instrument is constructed, the structurethereof being shown in FIG. 1. According to the requirement of radiationprotection, the shielding box is made of a material such as lead,tungsten alloy or the like having a proper thickness, thus ensuringpersonnel safety when the device is used.

The shielding box 1 is in the shape of a cube having a side length of 1m, and the geometric center thereof is set as a check point 12. Theincident hole 8 having the diameter of 120 mm and used for the incidenceof gamma rays is arranged in the geometric center of the side which isclose to the radiator (the radiation source 4), of the shielding box 1.The shielding box 1 is segmented into two parts by one plane that isperpendicular to the connecting line of the incident hole 8 and thecheck point 6 and that contains the check point 6, and the dose featurepoint 7 is located at the part close to the incident hole 8 on thebottom center line of the shielding box 1 and spaced 100 mm from theprojection point at the check point 6 on the bottom; the test hole 5having the diameter of 200 mm is arranged at the top of the shieldingbox 1, and is used for putting the probe of the instrument 2 to bedetected; scattering gamma ray spectra in the box are measured by usingan Inspector1000 portable gamma spectrometer of Canberra company, thereference point of the probe of the gamma spectrometer 9 is aligned withthe dose feature point 7 on the bottom of the shielding box 1, and theprobe of the gamma spectrometer 9 is fixed.

The activity degree of the ¹³⁷Cs radioisotope source is 1 Ci, theincident ray beam 3 is provided for the shielding box 1 via the device 4such as a radiator or the like, and the attenuation times of theincident ray beam 3 is adjusted according to the source strength of theradiation source and the range of the instrument to be detected. Thetimes of an attenuator is adjusted according to the range of the commongamma ray dose (rate) and dose equivalent (rate) instrument to obtainfive experiment source strengths V_(j),(j=1, 2, . . . , 5), and therange of the dose rate is 65 μGy/h−1.25 mGy/h.

An experiment is carried out according to the method of this embodiment,a prediction model of the air kerma conventional true value at the checkpoint 6 is obtained, and the specific implementation steps are asfollows:

Step A

A PTW-32005 standard graphite cavity ionization chamber is arranged atthe check point 6 of the shielding box, and the air kerma value at thecheck point 6 K_(j)′ when the source strength is V_(j) is measured.

Step B

Totally nine different types of common gamma ray dose (rate) instrumentsBH3103A, FJ317E, SSM-1, FD-3013B, CIT-2000FX·γ, Inspector1000(containing two kinds of probes: an IPRON-3 probe and an IPROS-2 probe)and Canberra Radiagem2000 are selected as samples for the experiment,and consecutively numbered as 1, 2 . . . 9, i.e., i=1 . . . 9.

Step C

The probe of the above No. 1 instrument is vertically arranged into theshielding box, and the reference point on the probe is superposed withthe check point 6. The source strength V_(j) is sequentially switchedfor measurement, and the readings of the No. 1 instrument and R_(1j) thegamma spectra S_(1j) recorded by the Inspector1000 are recorded. FIG. 2shows the scattering gamma spectra of the No. 1 instrument S_(1j) in theshielding box and under five source strengths V_(j).

Step D

The No. 1 instrument is arranged in the standard radiation field of“γ-ray air kerma (protection level) measurement standards” of anionizing radiation metrology station of China Academy of EngineeringPhysics to look for a point P_(ij) having the reading R_(1j). The airkerma conventional true value at the point P_(ij) is obtained accordingto the existing parameters of the standard radiation field, and thevalue is the air kerma conventional true value K_(1j) at the check point6 of the shielding box when the probe of the No. 1 instrument isarranged at the check point 6 of the shielding box under the sourcestrength V_(j) in step C.

Step E

The probes of the No. 2 to No. 9 instruments are respectively arrangedat the check point 6 in the shielding box, and 45 groups of K_(ij),S_(ij) and K_(j)′ data can be obtained by repeating steps C and D aboveunder five V_(j) conditions. The data has a function relationshipK_(ij)=f₁(S_(ij),K_(j)′), which is a mathematic prediction model forpredicting the air kerma conventional true value of the probe of theinstrument to be detected at the check point 6 of the shielding box inthe method of the present invention.

Step F

The S_(ij) is scattered according to an energy interval 3 keV to obtainan array of 512 counting rates η_(ijn) corresponding to the energy ofthe scattering gamma ray. According to S_(ij) features, in order toreduce the volume of calculating data, the first 250 counting rateshaving obvious features are selected as valid data, and 250-dimensionalvectors a_(ij) of the counting rates using the energy of the scatteringgamma ray as a research object are constructed; then a scattering gammaenergy spectrum counting rate data matrix sample Φ_(45×250) isconstructed via the experimental data of the probes of the nine gammadose (rate) instruments under the five source strength V_(j) conditions;the principal components of 45 pieces of 250-dimensional vectors a_(ij)are solved by adopting a principal component analysis (PCA) method,i.e., a covariance matrix ξ_(250×250) is obtained first from Φ_(45×250),and then 250 feature values λ₁≧λ₂≧ . . . ≧λ₂₅₀≧0 of the covariancematrix ξ_(250×250) and the corresponding feature vectors are solved. Thescore matrix of the principal components is T_(250×m)=(t₁, . . . ,t_(m)), wherein m is determined by formula Σ_(k=1) ^(m)λ_(k)≧δ_(m). Theprincipal component score matrix of the 250-dimensional vectors a_(ij)is T_(n×m)=(t₁, . . . , t_(m)), m≦n. When δ_(m) is 90%, m=2. The linearcombination coefficient of the score vectors t₁ and t₂ of two principalcomponents is shown as FIG. 3.

Step G

According to step F, a function relationship ψ_(ij)=f₂(S_(ij)) betweenψ_(ij) and S_(ij) can be obtained. The simulated prediction modelK_(ij)=f₁(S_(ij),K_(j)′) in step E can be simplified intoK_(ij)=f₃(ψ_(ij),K_(j)′) by replacing S_(ij) with ψ_(ij). Moreover, adata matrix sample (K_(ij),ψ_(ij),K_(j)′)_(45×(m+2)) can be obtained viaexperiments by using the nine gamma ray dose (rate) instruments underfive different radiation source strength V_(j) conditions.

Step H

Based on the data matrix sample (K_(ij),ψ_(ij),K_(j)′)_(45×(m+2))obtained via experiments, a prediction model K_(ij)=f₃(ψ_(ij),K_(j)′) ofK_(ij) is obtained by adopting a least squares support vector machine(LS-SVM, an improved form of SVM) regression method in this embodiment.

The prediction model is trained on a Matlab software platform for theWindows7 system, and the version of the Matlab software is 2012a. Aradial basis function

${K\left( {x,x_{i}} \right)} = {\exp\left( {- \frac{{{x - x_{i}}}^{2}}{2\; \sigma^{2}}} \right)}$

is selected as the kernel function of the model by calling a leastsquares support vector machine toolbox (LS-SVMlab Toolbox User's Guideversion 1.5) in the platform, and the parameter σ² of the kernelfunction and the regularization parameter c are determined by an L-foldcross validation method. L is set to be equal to 10, and the data sample(K_(ij),ψ_(ij),K_(j)′)_(45×(m+2)) is allocated to a training set and atest set according to a proportion of 6:3; and training is ended whenthe test error is less than or equal to 5%. The prediction model ofK_(ij) is K_(ij)=F[(ψ_(ij),K_(j)′), (ψ′,K″)]′×α+b is finally acquired,wherein F is the kernel function, a and b are parameters of the model,ψ_(ij) and K_(j)′ are respectively the principle component vector of theenergy spectrum S_(ij) when the instrument to be detected is introducedinto the shielding box and the air kerma value at the check point of theshielding box when no probe is introduced under the source strength, ψ′and K″ are sample data of the principle component vector of the energyspectrum for training the model and air kerma sample data at the checkpoint in the shielding box when no probe is introduced. In combinationwith the function ψ_(ij)=f₂(S_(ij)), the model can be expressed asK_(ij)=F[(f₂(S_(ij)),K_(j)′), (ψ′,K″)]′×αa+b, i.e.,K_(ij)=f₁(S_(ij),K_(j)′).

When the BH3103A gamma ray dose rate instrument 2 to be detected iscalibrated, a probe of the BH3103A is put into the shielding box, andthe reference point of the probe is superposed with the check point 6 ofthe MRR; a proper radiation source strength V_(j) is determinedaccording to the range of the BH3103A in a manner of selecting anattenuator or the like so that the reading of the BH3103A is nearby themidpoint of the calibration range, scattering gamma spectra measured bythe gamma spectrometer 9 are recorded, the principle component vectorψ_(i) of the spectrum data is extracted and introduced into theprediction model established K_(ij)=F[(ψ_(ij),K_(j)′), (ψ′,K″)]′×α+b,the air kerma conventional true value K_(ij) at the check point 6 of theMRR under such condition is 91.27 μGy/h, the mean R of readings of thefive instruments is 89.82 μGy/h, a calibration factor is obtainedaccording to formula

${\omega = {\frac{K_{ij}}{\overset{\_}{R}} - 1.016}},$

and calibration of the instrument is thus realized.

The aforesaid embodiment is only an example for realizing the presentinvention, and the present invention can be realized in multiple ways.For example, the shape of the small-scale reference radiation MRR is notlimited to a cube, the MRR in other shape such as a cuboid or the likedoes not influence the effect of the present invention, and the methodsof introducing gamma rays via the shielding box and limiting the gammarays into a small closed space are all implementations of the presentinvention; the check point and the dose feature point are not limited tothe positions in the embodiment, as long as they are located in the MRR,can fulfill the purposes required by the claims and do not influence theeffect of the present invention; as for the SVM method for establishingthe prediction model K_(ij)=f₁(S_(ij),K_(j)′) of the air kermaconventional true value at the check point in the MRR, the SVM methodhas multiple forms and is being rapidly developed, the SVM regressionmode is not limited to the least squares support vector machine LS-SVMused in this embodiment, and other modes of SVM, C-SVM, v-SVM and thelike adopting an SMO (Sequential Minimal Optimization) algorithm areavailable for fulfilling the purpose of establishing the predictionmodel K_(ij)=f₁(S_(ij),K_(j)′) of the air kerma conventional true valueat the check point in the MRR.

Other than one ¹³⁷Cs cesium source for calibration of the gamma ray dose(rate) instrument in this embodiment, ¹³⁷Cs, ²⁴¹Am and ⁶⁰Co sources andthe method introduced in the present invention can also besimultaneously adopted to obtain the indicators of energy response,angle response and the like of the gamma ray dose (rate) instrument. AnX ray machine serving as a ray source and the method of the presentinvention can also be adopted for verification and calibration of gammaand X ray dose (rate) instruments.

Although the content of the present invention has been introduced indetail via the above preferred embodiment, the above introduction shallnot be regarded as a limitation to the present invention. It would beobvious for a person having professional knowledge and skills to makevarious modifications, substitutions and avoidances to the presentinvention upon reading the above content. Therefore, the protectionscope of the present invention should be defined by the appended claims.

1. An air kerma conventional true value measuring method, comprising thefollowing steps: step 1, establishing a small-scale reference radiationfield, the small-scale reference radiation field comprises a shieldingbox comprising a side length not more than 1.5 meters, the shielding boxbeing positioned horizontally and an incident hole being provided on aside thereof for incidence of incident rays, a check point beingarranged in a direction of the incident rays in the shielding box, theshielding box being further provided with a test hole on an uppersurface through which a probe of an instrument to be detected can be putinto the shielding box, a reference point on the probe being superposedwith the check point, a dose feature point being also arranged in theshielding box, the shielding box being segmented into two parts by oneplane perpendicular to a connecting line of the incident hole and thecheck point and containing the check point, the dose feature point beinglocated at a part close to the incident hole in the shielding box and ata position not directly irradiated by the incident rays, a gammaspectrometer being arranged in the shielding box, a reference point on aprobe thereof being superposed with the dose feature point and the probebeing fixed in the shielding box; step 2, selecting a proper radiationsource and source strength to provide incident rays for the shieldingbox; step 3, selecting a plurality of gamma ray dose measurementinstruments as experimental samples for constructing a prediction modelto obtain the prediction model of the air kerma conventional true valueat the check point; and step 4, putting the probe of the instrument tobe detected at the check point, recording scattering gamma spectrameasured by the gamma spectrometer, and introducing the scattering gammaspectra to the prediction model to obtain an air kerma conventional truevalue.
 2. The air kerma conventional true value measuring method ofclaim 1, wherein step 3 comprises the following specific steps: step 31,selecting a plurality of gamma ray dose measurement instruments asexperimental samples for constructing a prediction model; step 32,measuring the air kerma conventional true value at the check point whenno experimental sample is put, putting a reference point of a probe ofan experimental sample on the check point, measuring the air kermaconventional true value at the check point by adopting an instrumenttransfer method, and acquiring gamma energy spectra of the dose featurepoint via the gamma spectrometer; step 33, acquiring a dose featurevalue by adopting a principal component analysis method according to thegamma energy spectra; and step 34, obtaining a prediction model of theair kerma conventional true value at the check point by adopting asupport vector machine regression method.
 3. The air kerma conventionaltrue value measuring method of claim 2, wherein step 32 comprises thefollowing specific steps: step 32A, putting a standard graphite cavityionization chamber at the check point, and measuring the air kermaconventional true value K_(j)′ at the check point when a strength of anincident ray beam is V_(j); step 32B, putting the reference point of theprobe of the i^(th) experimental sample at the check point, setting thestrength of the incident ray beam as V_(j), recording a reading of theexperimental sample R_(ij) and obtaining the gamma energy spectrumS_(ij) of the dose feature point at the moment via the gammaspectrometer; step 32C, putting the experimental sample in the standardreference radiation field to look for a point having the reading equalto R_(ij), the corresponding air kerma conventional true value of thepoint being the air kerma conventional true value K_(ij) at the checkpoint; and step 32D, sequentially putting the reference points of theprobes of the x experimental samples in the check point, and repeatingsteps 32A to 32C under y source strength conditions to obtain x×y groupsof K_(ij), S_(ij) and K_(j)′ data for constructing a model of a functionrelationship K_(ij)=f₁(S_(ij),K_(j)′).
 4. The air kerma conventionaltrue value measuring method of claim 3, wherein step 33 comprises thefollowing steps: step 33A, scattering each acquired S_(ij) according toa certain energy interval ΔE to obtain a counting rate η_(ijn) arraycorresponding to the energy of the scattering gamma ray, andconstructing n-dimensional vectors a_(ij) of counting rates using theenergy of the scattering gamma ray as a research object; step 33B,constructing a scattering gamma energy spectrum counting rate datamatrix sample Φ_((x×y)×n) via experiments of the probes of the xexperimental samples under the y source strength conditions in step 32D;step 33C, solving principal components of the n-dimensional vectorsa_(ij) by adopting a principal component analysis method to obtainprincipal component vectors ψ_(ij)=T_(n×m) ^(T)·a_(ij) of then-dimensional vectors a_(ij), m≦n, T_(n×m) ^(T) being a transposition ofT_(n×m), and T_(n×m) referring to obtaining a covariance matrix fromξ_(n×n) from Φ_((x×y)×n); and solving a score matrix composed of m firstfeature vectors of the covariance matrix ξ_(n×n); and step 33D,obtaining a function relationship ψ_(ij)=f₂(S_(ij)) between ψ_(ij) andS_(ij), thus obtaining K_(ij)=f₃(ψ_(ij),K_(j)′)=f₃[f₂(S_(ij)),K_(j)′].5. The air kerma conventional true value measuring method of claim 4,wherein in step 33A, the certain energy interval ΔE refers to:ΔE=1500/(128×2^(z))keV, 0≦z≦4, z being an integer.
 6. The air kermaconventional true value measuring method of claim 4, wherein step 33Ccomprises the following specific steps: step 33C1, obtaining acovariance matrix ξ_(n×n) from Φ_((x×y)×n), and solving n feature valuesλ₁≧λ₂≧ . . . ≧λ_(n)≧0 of the covariance matrix ξ_(n×n) and correspondingfeature vectors t₁, . . . , t_(m), . . . t_(n); step 33C2, obtaining ascore matrix T_(n×m)=(t₁, . . . , t_(m)) of the principal components,wherein m is determined by formula Σ_(k=1) ^(n)λ_(k)≧δ_(m), δ_(m)≧85%;and step 33C3, obtaining the principal component vectors ψ_(ij)=T_(n×m)^(T)·a_(ij) of the n-dimensional vectors a_(ij), wherein m≦n, andT_(n×m) ^(T) is a transposition of T_(n×m).
 7. The air kermaconventional true value measuring method of claim 6, wherein step 34comprises the following steps: step 34A, obtaining a data matrix sample(K_(ij),ψ_(ij),K_(j)′)_((x×y)×(m+2)) via the experiments of the probesof the x experimental samples under the y kinds of V_(j) conditions instep 32D, and obtaining a prediction model K_(ij)=f₃(ψ_(ij),K_(j)′) ofK_(ij) by adopting a support vector machine regression method.
 8. Theair kerma conventional true value measuring method of claim 7, whereinin step 34A, the specific method of obtaining a prediction modelK_(ij)=f₃(ψ_(ij),K_(j)′) of K_(ij) by adopting a support vector machineregression method comprises the steps of: training a kernel functionselected by a regression prediction model as a radial basis kernel, theparameter of the kernel function is determined by a cross validationmethod; when the model is constructed, allocating the data samples(K_(ij),ψ_(ij),K_(j)′)_((x×y)×(m+2)) to a training set and a test setaccording to a certain proportion; and when the test error is not morethan 5%, ending the training, and determining the prediction modelK_(ij)=f₃(ψ_(ij),K_(j)′).
 9. The air kerma conventional true valuemeasuring method of claim 8, wherein the certain proportion refers tothat the proportion of the training set to the test set is more than orequal to 1:1.
 10. The air kerma conventional true value measuring methodof claim 9, wherein step 4 comprises the following steps: step 41,putting the reference point of the probe of the instrument to bedetected at the check point; step 42, selecting a proper radiationsource and putting it into an isotope radiation source accommodatingdevice, and adjusting an attenuator to obtain the measured strengthV_(j) of the incident ray beam; and step 43, measuring the scatteringgamma spectrum at the moment by using the gamma spectrometer, andintroducing the scattering gamma spectrum data into the prediction modelconstructedK_(ij)=f₃(ψ_(ij),K_(j)′)=f₃[f₂(S_(ij),),K_(j)′]=f₁(S_(ij),K_(j)′) toobtain an air kerma conventional true value at the check point.
 11. Theair kerma conventional true value measuring method of claim 8, whereinstep 4 comprises the following steps: step 41, putting the referencepoint of the probe of the instrument to be detected at the check point;step 42, selecting a proper radiation source and putting it into anisotope radiation source accommodating device, and adjusting anattenuator to obtain the measured strength V_(j) of the incident raybeam; and step 43, measuring the scattering gamma spectrum at the momentby using the gamma spectrometer, and introducing the scattering gammaspectrum data into the prediction model constructedK_(ij)=f₃(ψ_(ij),K_(j)′)=f₃[f₂(S_(ij)),K_(j)′]=f₁(S_(ij),K_(j)′) toobtain an air kerma conventional true value at the check point.